Method and apparatus providing a cross-point memory array using a variable resistance memory cell and capacitance

ABSTRACT

The invention relates to a method and apparatus providing a memory cell array in which each resistance memory cell is connected in series to a capacitive element. Access transistors are not necessary to perform read and write operations on the memory cell. In one exemplary embodiment, the capacitive element is a capacitor.

FIELD OF THE INVENTION

The invention relates to memory structures utilizing variable resistance states for data storage.

BACKGROUND OF THE INVENTION

Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible, can be written or read very quickly, is non-volatile, but indefinitely alterable, and consumes little power. Emerging variable resistance memories increasingly offer these advantages. Chalcogenide-based memory using conductive metal or metal ions in a chalcogenide backbone is one example of such a memory. One such chalcogenide-based memory device is disclosed in U.S. Pat. No. 6,348,365, assigned to Micron Technology Inc. and incorporated herein by reference. Additionally, Magnetoresistive Random Access Memory (MRAM) technology has been increasingly viewed as offering these advantages. Other types of variable resistance memories include polymer-based memory and phase change chalcogenide-based memory.

A chalcogenide-based memory element, employing metal or metal ions, has a structure including a chalcogenide glass region incorporating a metal (or metal ions) and electrodes on either side of the glass region. Information can be stored as a digital “1” or “0” as stable resistance states. A typical chalcogenide glass used in chalcogenide-based memory devices is Ge_(x)Se_(100−x). The chalcogenide glass can also be used in conjunction with layers of Ag and/or Ag₂Se. The glass region of a chalcogenide-based memory element can be made less resistive upon application of a threshold write voltage. This less resistive state is maintained in a non- or semi-volatile manner and is reversible by applying a reversed voltage. The resistance state of a chalcogenide-based memory element can be sensed by the application of a sub-threshold voltage through the cell element.

A magnetic memory element has a structure which includes ferromagnetic layers separated by a non-magnetic barrier layer that forms a tunnel junction. An example of an MRAM device is described in U.S. Pat. No. 6,358,756 to Sandhu et al. Information can be stored as a digital “1” or a “0” as directions of magnetization vectors in resistance values which depend on the relative pinned and free ferromagnetic layers. Magnetic vectors in one ferromagnetic layer are magnetically fixed or pinned, while the magnetic vectors of the free ferromagnetic layer are not fixed so that the magnetization direction is free to switch between “parallel” and “antiparallel” states relative to the pinned layer. In response to parallel and antiparallel states, the magnetic memory element represents two different stable resistance states, which are read by the memory circuit as either a “1” or a “0.” Passing a current through the MRAM cell enables detection of the resistance states.

Polymer-based memory, another type of variable resistance memory, utilizes a polymer-based layer having ions dispersed therein or, alternatively, the ions may be in an adjacent layer. The polymer memory element is based on polar conductive polymer molecules. The polymer layer and ions are between two electrodes such that upon application of a voltage or electric field the ions migrate toward the negative electrode, thereby changing the resistivity of the memory cell. This altered resistivity can be sensed as a memory state.

Phase change chalcogenide memory, another type of variable resistance memory, switches resistivity states by undergoing a phase change in response to resistive heating. The two phases corresponding to the two stable resistivity states include a polycrystalline state and an amorphous state. The amorphous state is a higher resistance state. The resistance state can be read as stored data.

Typical resistive memory cell arrays employ access transistors to enable the reading and writing of the memory cells. However, power is wasted when an access transistor is used to change the resistance state of a resistance variable memory cell because the power is directed from the power source to ground. Therefore, the access transistor and memory circuit consumes power. Further, the use of an access transistor makes it difficult to stack the memory cells and attain a high memory cell density. A method and apparatus for accessing a variable resistance memory cell without the use of an access transistor is desirable.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a method and apparatus providing a memory cell array in which each resistance memory cell is connected in series to a capacitive element. Access transistors are not necessary to perform read and write operations on the memory cell. In one exemplary embodiment, the capacitive element is a capacitor.

BRIEF DESCRIPTION OF THE DRAWING

The above and other advantages and features of the invention will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings, in which:

FIG. 1 illustrates a portion of an exemplary memory array architecture according to an embodiment of the invention; and

FIG. 2 is a block diagram of a processor-based system incorporating a memory device constructed in accordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration of specific embodiments by which the invention may be practiced. It should be understood that like reference numerals represent like elements throughout the drawings. These exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention.

The invention relates to a novel array architecture for memory technology, particularly variable resistance memory with low volatility (also known as “non-volatile” in the art). The invention may be employed with chalcogenide-based memory employing metal or metal ions in a chalcogenide backbone. In such a variable resistance memory, the resistance of the memory cell changes as a threshold voltage is applied to the cell. The memory array architecture of the invention may also be used with other types of variable resistance memory. Further, the novel architecture replaces the access transistor typically need to read from and write to the memory cell with a small capacitance, such as provided by a small capacitor. A capacitor having a size and capacitance on the order of a capacitors used in DRAM memory cells may be employed. However, the invention is not limited to the use of a DRAM capacitor. For example, the parasitic capacitance of a conductor or structure having capacitance may also be used.

Now referring to the drawings, where like reference numbers designate like components of the invention, FIG. 1 shows a portion of a memory array 10 having a memory array architecture in accordance with an exemplary embodiment of the invention. The array 10 has memory cells 32 and capacitors 30, connected in series, which comprise a memory circuit 24. The memory cells 32 are variable resistance memory cells. Each memory cell 32 is electrically coupled to a respective column line 26 and each capacitor 30 is electrically coupled to a respective row line 28. The column lines 26 are electrically coupled to sense circuitry 34. The state of the memory cell 32 is either a high or low resistance state. For example, the low resistance may be approximately 10 kΩ and the high resistance may be approximately 10 kΩ.

The architecture of the illustrated embodiment provides for a substantial reduction in the power consumed by the memory cells by removing the transistors typically required to access the memory cells 32. Instead of an access transistor, a small capacitance (e.g., capacitor 30) is used to change the state of the memory cell during writes and to read-out the state of the cell during reads. The energy stored by the small capacitances provides the energy necessary to conduct the write and erase operations of the cell.

To read the memory cell 32, the column line 26 is set to a known starting potential, (e.g., the mid-point between Vdd and ground). A voltage is then applied to the capacitor 30 from the row line 28. For example, the voltage of the row line 28 may be switched from low to high, which couples the capacitance of capacitor 30 to the memory cell 32. Upon doing so, the column line 26 voltage will increase to approximately the same level regardless of the resistance state of the memory cell 32. However, the sense circuitry 34 connected to each column line 26, to determine the resistance of the memory cell 32, determines how fast the coupling occurs. The speed of the coupling determines the resistance state and thus, the logical state, of the cell 32. Various sensing circuits 34 can be used depending on the specific memory type (e.g., MRAM, chalcogenide-based memory, polymer memory, chalcogenide memory, or others) of the cells 32 of the array 10.

To write to the memory cell 32, the column line 26 is transitioned from the mid level voltage to a low voltage state, for example, at the same time the row line 28 is transitioned from a low to high voltage state; this applies a threshold voltage to the memory cell 32, which writes the memory cell 32 to a low resistance state. Conversely, to write the memory cell to a high resistance state, the column line 26 is kept at a mid level voltage, for example, while the row line 28 transitions from a low to high voltage. Then, the row line 28 is transitioned from a high to low voltage while the column line 26 transitions from the mid level voltage to a high level voltage, thereby applying enough energy to overwrite the memory cell 32.

It should be noted that voltage is applied to the memory cell 32 during both the read and write operations, accordingly, the voltage applied during the read operation must be maintained below the memory cells' 32 threshold voltage so that the memory cell 32 is not overwritten.

The invention is not limited to the read and write operations describe herein as they are only exemplary. The memory cell 32 may be read and written by a plurality of methods. The memory cell 32 is pre-conditioned for different threshold voltages, thereby the memory cell 32 may be read and written by applying the appropriate voltages. In addition, the sensing technique for sense amplifier 34 is not limited to a voltage sensing technique. For example, a current sensing technique may be used, although it consumes more power. Additionally, the sensing circuits illustrated in U.S. Pat. No. 6,888,771, herein incorporated by reference, may used, for example, in accordance with the embodiments of this invention.

As previously noted, the memory cells 32 of the invention may be MRAM, chalcogenide-based memory using metal or metal ions, polymer-based, phase-changing chalcogenide-based, and other non-volatile type memory cells. In a desired embodiment, chalcogenide-based memory employing metal or metal ions is used as the memory cells 32 due to their low power consumption.

The capacitance of the invention may be provided by a discrete capacitor 30, a conductive line (e.g., words or bit lines) with a high enough parasitic capacitance or by another structure which can provide a capacitance. Such memory cells 32 and capacitors 30 may be fabricated as is known in the art. Interconnect lines, such as column lines and row lines, can be made of materials and fabricated as is known in the art. Further, since no access transistors are required, layers of memory cells 32 may be stacked over one another.

FIG. 2 illustrates an exemplary processing system 900 which may utilize a memory device 100 having a memory array 10 in accordance with the invention. The processing system 900 includes one or more processors 901 coupled to a local bus 904. A memory controller 902 and a primary bus bridge 903 are also coupled the local bus 904. The processing system 900 may include multiple memory controllers 902 and/or multiple primary bus bridges 903. The memory controller 902 and the primary bus bridge 903 may be integrated as a single device 906.

The memory controller 902 is also coupled to one or more memory buses 907. Each memory bus 907 accepts memory components 908 which include at least one memory device 100 of the present invention. The memory components 908 may be a memory card or a memory module. Examples of memory modules include single inline memory modules (SIMMs) and dual inline memory modules (DIMMs). The memory components 908 may include one or more additional devices 909. For example, in a SIMM or DIMM, the additional device 909 might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller 902 may also be coupled to a cache memory 905. The cache memory 905 may be the only cache memory in the processing system. Alternatively, other devices, for example, processors 901 may also include cache memories, which may form a cache hierarchy with cache memory 905. If the processing system 900 includes peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller 902 may implement a cache coherency protocol. If the memory controller 902 is coupled to a plurality of memory buses 907, each memory bus 907 may be operated in parallel, or different address ranges may be mapped to different memory buses 907.

The primary bus bridge 903 is coupled to at least one peripheral bus 910. Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus 910. These devices may include a storage controller 911, a miscellaneous I/O device 914, a secondary bus bridge 915, a multimedia processor 918, and a legacy device interface 920. The primary bus bridge 903 may also coupled to one or more special purpose high speed ports 922. In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system 900.

The storage controller 911 couples one or more storage devices 913, via a storage bus 912, to the peripheral bus 910. For example, the storage controller 911 may be a SCSI controller and storage devices 913 may be SCSI discs. The I/O device 914 may be any sort of peripheral. For example, the I/O device 914 may be a local area network interface, such as an Ethernet card. The secondary bus bridge 915 may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge 915 may be an universal serial port (USB) controller used to couple USB devices 917 via to the processing system 900. The multimedia processor 918 may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional devices such as speakers 919. The legacy device interface 920 is used to couple legacy devices, for example, older styled keyboards and mice, to the processing system 900.

The processing system 900 illustrated in FIG. 2 is only an exemplary processing system with which the invention may be used in a memory array. While FIG. 2 illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system 900 to become more suitable for use in a variety of applications. For example, many electronic devices which require processing may be implemented using a simpler architecture which relies on a CPU 901 coupled to memory components 908 and/or memory devices 100. These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system (GPS) and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.

It should be appreciated that other embodiments of the invention include the methods of fabricating the circuits of the invention. For example, in one exemplary embodiment, a method of fabricating the memory array comprises providing a plurality of memory circuits organized into a plurality of rows and columns, each circuit comprising a variable resistance memory cell electrically connected in series with a capacitance, said memory circuit being connected between a column line and a row line.

The processes and devices described above are merely illustrative of but a few of the preferred methods and typical devices that could be used and produced in accordance with the invention. The above description and drawings illustrate embodiments, which achieve the objects, features, and advantages of the present invention. However, it is not intended that the present invention be strictly limited to the above-described and illustrated embodiments. For example, the read and write actions described herein are only exemplary. The invention may be read/written/erased in any one of variety of ways by changing the applied and starting voltages. Any modifications of the present invention that come within the spirit and scope of the following claims should be considered part of the present invention. 

1-20. (canceled)
 21. A method of reading a resistive memory cell in a memory circuit, the memory cell being directly coupled to a first interconnect line and being coupled to a second interconnect line via a capacitative element, the method comprising: setting the first interconnect line to a first known voltage; changing a voltage of the second interconnect line from a first applied voltage to a second applied voltage such that a difference between the first known voltage and the second applied voltage is less than a threshold write voltage for the memory cell; and determining a resistance state of the memory cell.
 22. The method of reading a resistive memory cell of claim 21, wherein the resistance state of the memory cell is determined by measuring an amount of time required for the voltage of the first interconnect line to approach the voltage of the second interconnect line.
 23. The method of reading a resistive memory cell of claim 22, wherein said amount of time is measured from a first time when the voltage of the second interconnect line is changed to the second applied voltage to a second time when the voltage of the first interconnect line approaches the voltage of the second interconnect line.
 24. The method of reading a resistive memory cell of claim 23, wherein the second time is sensed by a voltage sensing circuit.
 25. The method of reading a resistive memory cell of claim 23, wherein the second time is sensed by a current sensing circuit.
 26. The method of reading a resistive memory cell of claim 21, wherein the first known voltage is between the first and second applied voltages.
 27. The method of reading a resistive memory cell of claim 21, wherein the first applied voltage is a logical low voltage and the second applied voltage is a logical high voltage.
 28. A method of writing to a resistive memory cell in a memory circuit, the memory cell being directly coupled to a first interconnect line and being coupled to a second interconnect line via a capacitative element, the method comprising: changing a voltage of the second interconnect line from a first applied voltage to a second applied voltage while changing a voltage of the first interconnect line from a third applied voltage to the first applied voltage, wherein the third applied voltage is in between the first and second applied voltages and a difference between the first and second applied voltages is greater than a threshold write voltage for the memory cell.
 29. The method of writing to a resistive memory cell of claim 28, wherein the memory cell is written to a low resistance state when the first applied voltage is a logical low voltage and the second applied voltage is a logical high voltage.
 30. The method of writing to a resistive memory cell of claim 28, wherein the memory cell is written to a high resistance state when the first applied voltage is a logical high voltage and the second applied voltage is a logical low voltage.
 31. A method of operating a resistive memory cell in a memory circuit, the memory cell being directly coupled to a first interconnect line and being coupled to a second interconnect line via a capacitative element, the method comprising: reading the memory cell by sensing an amount of time required for a voltage of the first interconnect line to approach a voltage of the second interconnect line when the voltage of the second interconnect line is changed from a first applied voltage to a second applied voltage, where a difference between the voltage of the first interconnect line and the second applied voltage is less than a threshold write voltage for the memory cell; and writing to the memory cell by changing the voltage of the second interconnect line from the first applied voltage to the second applied voltage while changing the voltage of the first interconnect line from a third applied voltage to the first applied voltage, wherein the third applied voltage is in between the first and second applied voltages and a difference between the first and second applied voltages is greater than the threshold write voltage for the memory cell.
 32. The method of operating a resistive memory cell of claim 31, wherein said amount of time is measured from a first time when the voltage of the second interconnect line is changed to the second applied voltage to a second time when the voltage of the first interconnect line approaches the voltage of the second interconnect line.
 33. The method of operating a resistive memory cell of claim 32, wherein the second time is sensed by a voltage sensing circuit.
 34. The method of operating a resistive memory cell of claim 32, wherein the second time is sensed by a current sensing circuit.
 35. The method of operating a resistive memory cell of claim 31, wherein the memory cell is written to a low resistance state when the first applied voltage is a logical low voltage and the second applied voltage is a logical high voltage.
 36. The method of operating a resistive memory cell of claim 31, wherein the memory cell is written to a high resistance state when the first applied voltage is a logical high voltage and the second applied voltage is a logical low voltage. 